I think they say that the small shift towards warmer weather caused a cascading effect. In other words, the small amount of warmer weather in turn caused this beetle to flourish which in turn drives the weather up even more.
I may have beech front property within my life time yet!
Mountain pine beetle is a native species, but climate change (warmer winters) has resulted in a population explosion and this beetle has been devastating western North American pine forests. It is an increasingly common example of a native species that has become invasive due to climate change. It is marching eastward and could eventually devastate pine forests across the continent.
And not just the pine forests. We have been bombarded by news media stories back here in BC for years now. The scariest of those stories was one saying there is some evidence the beetle can and has jumped species. When faced with no pine close enough to survive they can and will burrow into other species of trees.